Added custom messages for conflicts directive. fixes #4965 (#5083)

Users can now add an optional custom message to the conflicts directive.
Layout on screen has been changed to improve readability and the long
spec is shown in tree format. Two conflicts in `espresso` have been
modified to showcase the feature.

var/spack/repos/builtin/packages/espresso/package.py

@@ -75,8 +75,17 @@ class Espresso(Package):
     patch('dspev_drv_elpa.patch', when='@6.1 ^elpa@2016.05.003')
 
     # We can't ask for scalapack or elpa if we don't want MPI
-    conflicts('+scalapack', when='~mpi')
-    conflicts('+elpa', when='~mpi')
+    conflicts(
+        '+scalapack',
+        when='~mpi',
+        msg='scalapack is a parallel library and needs MPI support'
+    )
+
+    conflicts(
+        '+elpa',
+        when='~mpi',
+        msg='elpa is a parallel library and needs MPI support'
+    )
 
     # Elpa is formally supported by @:5.4.0, but QE configure searches
     # for it in the wrong folders (or tries to download it within